Cell – The Unit of Life





Chapter: The Fundamental Unit of Life

Introduction: Discovery of Cell

In 1665, Robert Hooke observed a thin slice of cork under a self-designed microscope. He noticed that cork looked like a honeycomb made of many small compartments.

๐Ÿ”น Cork is obtained from the bark of a tree
๐Ÿ”น These compartments were empty because cork cells were dead
๐Ÿ”น Hooke named these compartments “cells”

Cell is a Latin word meaning “a little room”




Importance of Hooke’s Discovery

  • First evidence that living organisms are made of small structural units
  • Laid the foundation of Cell Biology
  • The term cell is still used today

What are Living Organisms Made Up of?

All living organisms—plants, animals, and microorganisms—are made up of cells.

Some organisms consist of:

  • One cell → unicellular organisms
  • Many cells → multicellular organisms

Activity 5.1 – Observation of Onion Peel Cells

Aim

To observe the structure of cells in an onion peel under a microscope.

Materials Required

  • Onion bulb
  • Forceps
  • Watch glass with water
  • Glass slide
  • Cover slip
  • Safranin solution
  • Compound microscope

Procedure

  1. Take a small piece of onion bulb.
  2. Peel off the inner epidermal layer using forceps.
  3. Place the peel in water (prevents drying and folding).
  4. Transfer the peel onto a glass slide with a drop of water.
  5. Add safranin stain to colour the cells.
  6. Place a cover slip carefully (avoid air bubbles).
  7. Observe under low power, then high power microscope.

Observation




  • Many rectangular, brick-like structures are seen.
  • Each structure is:
    • Surrounded by a cell wall
    • Contains a nucleus
    • Filled with cytoplasm
  • Cells are arranged closely and regularly

What is a Cell?


A cell is the smallest living unit capable of performing all vital life processes such as metabolism, growth, responsiveness, and reproduction.

 Definition 

A cell is the basic structural, functional, and biological unit of all living organisms.

๐Ÿ”น Structural unit → Forms tissues and organs

๐Ÿ”น Functional unit → Site of metabolism

๐Ÿ”น Biological unit → Carries hereditary material (DNA


Unicellular and Multicellular Organisms

Unicellular Organisms

Organisms made up of a single cell, which performs all life activities.

 Examples:

  • Amoeba
  • Chlamydomonas
  • Paramecium
  • Bacteria

Multicellular Organisms

Organisms made up of many cells, with division of labour.

 Examples:

  • Plants
  • Animals
  • Some fungi

Important Historical Contributions

Scientist Contribution
Robert Hooke (1665) Discovered cells in cork
Leeuwenhoek (1674) First observed living cells
Robert Brown (1831) Discovered nucleus
Purkinje (1839) Coined term “protoplasm”
Schleiden & Schwann Cell theory
Virchow (1855) Cells arise from cells


Cell Theory


Proposed jointly by Matthias Schleiden (1838) and Theodor Schwann (1839), and later modified by Rudolf Virchow (1855).


Postulates of Cell Theory


1. All living organisms are composed of one or more cells.



2. The cell is the basic unit of structure and function of life.



3. All cells arise from pre-existing cells (Omnis cellula e cellula).




๐Ÿ“Œ Modern Cell Theory (Additions):


Energy flow (metabolism) occurs within cells.


Cells contain hereditary information passed to daughter cells.


All cells have a similar basic chemical composition.




---


Exceptions to Cell Theory


Exception Reason


Viruses Non-cellular, metabolically inactive outside host

Striated muscle fibres Multinucleated single cell

Aseptate fungal hyphae Coenocytic (no septa, many nuclei)

Red blood cells (mammals) Lack nucleus (functional exception)




---


2. Historical Discoveries in Cell Biology


Year Scientist Contribution


1665 Robert Hooke Observed dead cork cells; coined term “cell”

1674 A. van Leeuwenhoek Observed living cells (protozoa, bacteria)

1838 Schleiden Plants are composed of cells

1839 Schwann Animals are composed of cells

1855 Rudolf Virchow Cells arise from pre-existing cells



๐Ÿ“Œ Key Insight:

Hooke observed cell walls, not living protoplasm.



---


3. Types of Cells


A. Prokaryotic Cells


Primitive cells lacking a true nucleus.


Characteristics


No nuclear membrane → DNA lies in nucleoid


No membrane-bound organelles


Cell wall usually present (peptidoglycan)


Ribosomes: 70S


Metabolism occurs in cytoplasm and plasma membrane


Reproduction: Binary fission



Examples


Bacteria


Cyanobacteria (Blue-green algae)


Mycoplasma (smallest free-living cell)



๐Ÿ“Œ Size: 1–10 ฮผm



---


B. Eukaryotic Cells


Advanced cells with structural compartmentalization.


Characteristics


True nucleus with nuclear envelope


Membrane-bound organelles present


Cytoplasmic ribosomes: 80S


Mitochondria & chloroplast ribosomes: 70S


Complex cytoskeleton


Sexual reproduction common



Examples


Amoeba


Plant cells


Animal cells



๐Ÿ“Œ Size: 5–100 ฮผm



---


4. Cell Size, Shape, and Number


Cell Size


Category Example


Smallest cell Mycoplasma (0.3 ฮผm)

Largest cell Ostrich egg

Longest cell Nerve cell (up to 1 m)



๐Ÿ“Œ Why small cells are efficient?

High Surface Area : Volume (SA:V) ratio → efficient diffusion.



---


Cell Shape (Functional Adaptation)


Cell Shape Function


RBC Biconcave Efficient O₂ transport

Neuron Long, branched Impulse conduction

Muscle cell Spindle-shaped Contraction

Guard cells Bean-shaped Regulate stomata

WBC Amoeboid Phagocytosis




---


Cell Number


Unicellular: Amoeba, Paramecium


Multicellular: Humans, plants



๐Ÿ“Œ In unicellular organisms, one cell performs all life functions.



---


5. Cell Structure and Organelles



---


Plasma Membrane (Cell Membrane)


Structure


Fluid Mosaic Model (Singer & Nicolson)


Phospholipid bilayer + proteins


Cholesterol (in animals) adds stability



Functions


Selective permeability


Cell recognition


Transport


Signal reception




---


Transport Across Membrane


Passive Transport


Diffusion


Osmosis


Facilitated diffusion (carrier proteins)



Active Transport


Against concentration gradient


Requires ATP


Example: Na⁺/K⁺ pump



Bulk Transport


Endocytosis


Phagocytosis (solids)


Pinocytosis (liquids)



Exocytosis




---


Cell Wall


Present in plants, fungi, bacteria


Composition:


Plants → Cellulose


Fungi → Chitin


Bacteria → Peptidoglycan




๐Ÿ“Œ Function:

Protection, rigidity, prevents osmotic bursting



---


Cytoplasm


Semi-fluid matrix


Contains enzymes, organelles


Site of metabolic reactions




---


Nucleus


Double membrane with pores


Contains chromatin (DNA + histones)


Nucleolus: rRNA synthesis



๐Ÿ“Œ Controls heredity and metabolism



---


Endoplasmic Reticulum (ER)


Type Function


RER Protein synthesis

SER Lipid synthesis, detoxification, Ca²⁺ storage




---


Ribosomes


Non-membranous


Protein synthesis


70S (prokaryotes, mitochondria)


80S (eukaryotes)




---


Golgi Apparatus


Cis → Medial → Trans faces


Modifies, packages proteins


Forms lysosomes




---


Lysosomes


Acidic vesicles


Contain hydrolytic enzymes


Autophagy, autolysis



๐Ÿ“Œ Called “Suicide bags”



---


Mitochondria


Double membrane


Inner membrane → cristae


Matrix contains DNA, ribosomes



๐Ÿ“Œ Powerhouse of the cell

๐Ÿ“Œ Semi-autonomous organelle



---


Plastids (Plants)


Type Function


Chloroplast Photosynthesis

Chromoplast Pigments

Leucoplast Storage




---


Vacuoles


Large in plants


Maintains turgor pressure


Storage & detoxification




---


Centrosome & Centrioles


Animal cells


Spindle fibre formation


9+0 microtubule arrangement




---


Cytoskeleton


Microtubules


Microfilaments


Intermediate filaments



๐Ÿ“Œ Maintains shape and movement



---


Peroxisomes & Glyoxysomes


Oxidative metabolism


Detoxification


Fat → sugar conversion (germinating seeds)




---


6. Transport Mechanisms


Diffusion


Movement from high → low concentration


Osmosis


Movement of water through semi-permeable membrane


Tonicity Effects


Solution Effect


Hypotonic Cell swells

Hypertonic Cell shrinks

Isotonic No change




---


Plasmolysis


Shrinkage of protoplast in hypertonic solution



---


Imbibition


Water absorption by hydrophilic substances



---


7. Cell Division


Cell Cycle


G₁ → S → G₂ → M




---


Mitosis


Equational division


Growth and repair


Two identical diploid cells




---


Meiosis


Reduction division


Gamete formation


Genetic variation




---


8. Differences Between Plant and Animal Cells


(Expanded conceptual understanding already included)



---


9. NCERT-Based Advanced Additions


Protoplasm


Living content of cell

= Cytoplasm + Nucleus



---


Chromosomes


DNA + proteins


Humans: 46 (23 pairs)


Sex chromosomes: XX / XY




---


Cell as a Metabolic Unit


Anabolism + Catabolism


Compartmentalization increases efficiency




---


Surface Area to Volume Ratio


Determines exchange efficiency


Explains why cells are microscopic




---


Endocytosis in Amoeba


Pseudopodia formation


Food vacuole → lysosome digestion




---


Conclusion


✔ Cell is the fundamental unit of life

✔ Structural complexity → functional efficiency

✔ Understanding cells explains growth, disease, heredity, and evolution



---


If you want, I can also provide:


Ultra-short revision notes


NEET-level MCQs with explanations


Diagrams with labels


Assertion-Reason questions


One-page summary sheet



Just tell me ๐ŸŒฑ

I am a dynamic research student, dedicated to mentoring numerous students for competitive exams. My achievements include qualifying in various prestigious exams such as IIT-JAM, CUET, NTSE, EF-SET, and SAT, alongside holding degrees in Physics, Nuclear Engineering, Psychology, and Spectroscopy. With training from esteemed institutions like ISRO and UNICEF, I have also amassed over 150 certifications from top universities like Yale, Stanford, and Microsoft.Beyond my academic and technical prowess, I excel in AI and digital marketing, proficient in AI skills, Python, R, and more. Moreover, I bring a wealth of experience in leadership, communication, problem-solving, and project management, contributing to my well-rounded and accomplished professional profile.

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